1. Field of the Invention
The present invention relates to a neutral control device of an automatic transmission that establishes a neutral condition by slipping and engaging a forward-drive engaging element while an accelerator pedal is being released just before the start or stop of the vehicle.
2. Description of the Prior Art
A known example of neutral control device of an automatic transmission is disclosed in Japanese laid-open Patent Publication No. 61-85225. In this known device, as shown in FIG. 14, a direct driven type valve that is driven by a low clutch pressure duty solenoid so as to control a low clutch pressure is provided in a hydraulic circuit that connects a shift valve, and a low clutch that is engaged when a hydraulic pressure is supplied from the shift valve so as to place the transmission in at least the 1st-speed gear position in the forward-drive range.
In the known neutral control device of the automatic transmission as described above, however, the low clutch pressure is controlled by the direct drive type valve in a range of oil pressure level from zero to the maximum level (line pressure), and therefore the gain of the oil pressure becomes considerably large, which tends to cause vibrations or variations in the oil pressure. Thus, the direct driven type valve is not suitable for use in neutral control that requires subtle control of the low clutch pressure.
The known neutral control device has another problem; if the low clutch pressure duty solenoid locks, or fails, while the direct driven type valve is placed in a drain position, the low clutch fails to be engaged in the forward-drive 1st-. 2nd-, and 3rd-gear positions in which it should be engaged, and the vehicle is unable to run due to racing of the engine.
In view of the above problems, it is proposed to provide a neutral control device that performs neutral control by using an oil pressure obtained from a low clutch control valve using the line pressure as an original pressure, as shown in FIG. 15, instead of using the pressure of the shift valve.
Although this neutral control device solves the above-described problem, two valves and two solenoids are newly added so as to perform neutral control, which is undesirable in view of the cost, and the addition of these valves makes it difficult to lay out the circuit including the shift valve.
Since the low clutch control valve uses the line pressure as the original pressure, as described above, the oil pressure may be supplied from the low clutch control valve to the low clutch due to a failure in the solenoid even where the neutral position is selected, thus allowing the vehicle to run in the N range.